Even in the best of times, Pakistan’s nuclear-weapons program warrants alarm. But these are perilous days. At a moment of unprecedented misgiving between Washington and Islamabad, new evidence suggests that Pakistan’s nuclear program is barreling ahead at a furious clip.

According to new commercial-satellite imagery obtained exclusively by Newsweek, Pakistan is aggressively accelerating construction at the Khushab nuclear site, about 140 miles south of Islamabad. The images, analysts say, prove Pakistan will soon have a fourth operational reactor, greatly expanding plutonium production for its nuclear-weapons program.

Unlike Iran, which has yet to produce highly enriched uranium, or North Korea, which has produced plutonium but still lacks any real weapons capability, Pakistan is significantly ramping up its nuclear-weapons program. Eric Edelman, undersecretary of defense in the George W. Bush administration, puts it bluntly: “You’re talking about Pakistan even potentially passing France at some point. That’s extraordinary.”

Pakistani officials say the buildup is a response to the threat from India, which is spending $50 billion over the next five years on its military. “But to say it’s just an issue between just India and Pakistan is divorced from reality,” says former senator Sam Nunn, who co-chairs the Nuclear Threat Initiative. “The U.S. and Soviet Union went through 40 years of the Cold War and came out every time from dangerous situations with lessons learned. Pakistan and India have gone through some dangerous times, and they have learned some lessons. But not all of them. Today, deterrence has fundamentally changed. The whole globe has a stake in this. It’s extremely dangerous.”

It’s dangerous because Pakistan is also stockpiling fissile material, or bomb fuel. Since Islamabad can mine uranium on its own territory and has decades of enrichment know-how—beginning with the work of nuclear scientist A. Q. Khan—the potential for production is significant.

Although the White House declined to comment, a senior U.S. congressional official who works on nuclear issues told Newsweek that intelligence estimates suggest Pakistan has already developed enough fissile material to produce more than 100 warheads and manufacture between eight and 20 weapons a year. “There’s no question,” the official says, “it’s the fastest-growing program in the world.”

The White House appears to have made a tacit tradeoff with Islamabad: for your cooperation in Afghanistan, we’ll leave you to your own nuclear devices.

What has leaders around the world especially worried is what’s popularly known as “loose nukes”—nuclear weapons or fissile material falling into the wrong hands. “There’s no transparency in how the fissile material is handled or transported,” says Mansoor Ijaz, who has played an active role in back-channel diplomacy between Islamabad and New Delhi. “And the amount—they have significant quantities—is what’s so alarming.”

A Defense Department official told Newsweek that the U.S. government is “confident that Pakistan has taken appropriate steps toward securing its nuclear arsenal.” But beyond palliatives, few in Washington want to openly discuss the nightmare scenario of terrorists getting hold of nuclear material or weapons. “The less that is said publicly, the better,” says Stephen Hadley, national-security adviser to President George W. Bush. “But don’t confuse the lack of public discussion for a lack of concern.”

The bomb lends the Pakistanis a certain diplomatic insouciance. Nukes, after all, are a valuable political tool, ensuring continued economic aid from the United States and Europe. “Pakistan knows it can outstare” the West, says Pakistani nuclear physicist Pervez Hoodbhoy. “It’s confident the West knows that Pakistan’s collapse is too big a price to pay, so the bailout is there in perpetuity. It’s the one thing we’ve been successful at.”

Pakistani leaders defend their weapons program as a strategic necessity: since they can’t match India’s military spending, they have to bridge the gap with nukes. “Regretfully, there are several destabilizing developments that have taken place in recent years,” Khalid Banuri of Pakistan’s Strategic Plans Division, the nuclear arsenal’s guardian, wrote in response to Newsweek questions. Among his country’s concerns, Banuri pointed to India’s military buildup and the U.S.’s -civilian nuclear deal with India.

“Most Pakistanis believe the jihadist scenario is something that the West has created as a bogey,” says Hoodbhoy, “an excuse, so they can screw us, defang, and denuclearize us.”

“Our program is an issue of extreme sensitivity for every man, woman, and child in Pakistan,” says former Pakistani president Pervez Musharraf, adding that the nukes are “well dispersed and protected in secure locations.” When asked whether the U.S. has a role to play in securing the arsenal, Musharraf said: “A U.S. role to play? A U.S. role in helping? Zero role. No, sir. It is our own production … We have not and cannot now have any intrusion by any element in the U.S.” To guard its “strategic assets,” Pakistan employs two Army divisions—about 18,000 troops—and, as Musharraf drily puts it, “If you want to get into a firefight with the forces guarding our strategic assets, it will be a very sad day.”

For now, the White House appears to have made a tacit tradeoff with Islamabad: for your cooperation in Afghanistan, we’ll leave you to your own nuclear devices. “People bristle at the suggestion, but it follows, doesn’t it?” says Rolf Mowatt-Larssen, formerly the CIA’s chief officer handling terrorism and weapons of mass destruction. “The irony is that the U.S. presence in Afghanistan and Pakistan, and the money we’re giving them to fight terrorism, could inadvertently aggravate the very problem we’re trying to stop. After all, terrorism and nukes is the worst-case scenario.”

With this fourth nuclear facility at Khushab coming online as early as 2013, and the prospect of an accelerated nuclear-weapons program, the U.S. is facing a diplomatic dilemma. “The Pakistanis have gone through a humiliation with the killing of Osama bin Laden,” says Nunn. “That’s never a time to corner somebody. But with both recent and preexisting problems, we are in a race between cooperation and catastrophe. Both sides need to take a deep breath, count to 10, and find a way to cooperate.”

Andrew Bast is a senior articles editor for the international edition of Newsweek. He has reported from four continents for several newspapers, including The New York Times, and now writes about global security. Follow him on Twitter: @andrewbast